The present invention relates to the production of pure amorphous silica from silica-based materials ( greater than 90% silica) while recycling reagents used in the process excepting the silica-based raw material.
Amorphous silica is mainly produced by acidulation of a soluble silicate, commonly by addition of sulfuric or hydrochloric acid to a sodium silicate (water glass) solution. The products produced by this method are termed precipitated silica or silica gel depending upon the details of the neutralization of the silicate solution by the acid. An additional method for producing an amorphous silica is by means of reaction of silicon tetrachloride or of a silicon alkoxide with hydrogen and oxygen, forming fumed silica and a byproduct such as HCl when starting from the chloride.
Silica produced by neutralization of a soluble silicate and fumed silica are invariably amorphous, if not subsequently heated above temperatures at which crystalline silica begins to form ( greater than 800 C.). Crystaline silica has been deemed a cancer suspect agent by inhalation, hence it is advantageous to produce an amorphous silica powder. Crystalinity is defined as the presence of a crystalline form of silica, quartz, cristobalite or tridamite, at levels in excess of 0.1% as detected by powder x-ray diffraction. The level of 0.1% being the accepted limit of detection for crystalline silica in amorphous silica.
Methods for preparation of amorphous silica from soluble silicates are well established, but suffer from the disadvantage that a soluble alkalai silicate and mineral acid are consumed in the production process.
A method in which an aluminosilicate is mixed with calcium chloride, baked at an elevated temperature (calcined) and leached with hydrochloric acid has been proposed in U.S. Pat. No. 1,868,499, however silica is considered an unwanted byproduct of alumina production in this patent. In PCT WO 97/22554 the authors propose a method whereby an aluminosilicate is calcined with CaCl2 and both an alumina and a silica product are produced. No indication is given as to the amorphous nature of the silica.
When using an aluminosilicate raw material, silica is only a fraction of the raw material, and the metal chlorides must be dealt with.
Amorphous silica has also been produced in U.S. Pat. No. 4,689,315, by autoclaving a siliceous material and a source of calcium such as quicklime and afterwards leaching the calcium silicate with an acid. This approach suffers from the drawback that the lime and HCl are consumed in the process.
A primary objective of this invention is to provide a method for the production of amorphous high surface silica directly from a quartz while recycling all reagents used in the process excepting the silica-based raw material.
An additional objective is to produce amorphous porous silica without the need for a soluble silicate source, and a mineral acid, which is consumed in the process.
An additional objective is to produce pure amorphous porous silica from waste materials, both crystalline and non-crystalline.